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Linux Kernel

Linux Kernel. Introduction to Memory Management 黃偉修. Outline. Virtual memory model Caches Page allocation and Deallcation Swapping Out and Discarding pages. Virtual memory model.

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Linux Kernel

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  1. Linux Kernel Introduction to Memory Management 黃偉修

  2. Outline • Virtual memory model • Caches • Page allocation and Deallcation • Swapping Out and Discarding pages

  3. Virtual memory model Why virtual memory ? . Large address Space . Protection . Shared memory <ex.shell program ,library> how to virtual memory ? . Page . Linear address transform . Demand paging <see figure>

  4. Virtual memory model • Linux Page .There level page Tables <see next figure> . Independent to processor - X86 2-level - Alpha 3-leveleach platform must provide translation macros that allow the kernel to traverse the page tables for a particular process

  5. Virtual memory model • Address Transfer . Ex:386 processor<see next table> A 32-bit Linear address is divided as follows: 31 ...... 22 21 ...... 12 11 ...... 0 DIR TABLE OFFSET

  6. Physical address is then computed (in hardware) as: CR3 + DIR points to the table_base. table_base + TABLE points to the page_base. physical_address page_base + OFFSET

  7. Format for Page directory and Page table entry: 31 ...... 12 11 .. 9 8 7 6 5 4 3 2 1 0 ADDRESS OS 0 0 D A 0 0 U/S R/W P D 1 means page is dirty (undefined for page directory entry). R/W 0 means readonly for user. U/S 1 means user page. P 1 means page is present in memory. A 1 means page has been accessed (set to 0 by aging). OS bits can be used for LRU etc, and are defined by the OS. I.e :When a page is swapped, bits 1-31 are used to mark where a page is stored in swap (bit 0 must be 0).

  8. Virtual memory model • Memory Mapping . The link of an image into a processes virtual address space is known as mapping . Every process virtual memory is represent by an mm_struct data structure - information about image- points to a number of vm_area_structdata struct . Vm_area_struct<see figure and detail>

  9. mm_struct the mm_struct data structure is used to describe the virtual memory of a task or process. struct mm_struct { int count; pgd_t * pgd; unsigned long context; unsigned long start_code, end_code, start_data, end_data; unsigned long start_brk, brk, start_stack, start_mmap; unsigned long arg_start, arg_end, env_start, env_end; unsigned long rss, total_vm, locked_vm; unsigned long def_flags; struct vm_area_struct * mmap; struct vm_area_struct * mmap_avl; struct semaphore mmap_sem; };

  10. vm_area_structEach vm_area_struct data structure describes an area of virtual memory for a process. struct vm_area_struct { struct mm_struct * vm_mm; /* VM area parameters */ unsigned long vm_start; unsigned long vm_end; pgprot_t vm_page_prot; 保護屬性unsigned short vm_flags; 存取權限和哪些保護屬性可設定/* AVL tree of VM areas per task, sorted by address */ short vm_avl_height; struct vm_area_struct * vm_avl_left; struct vm_area_struct * vm_avl_right;/* linked list of VM areas per task, sorted by address */ struct vm_area_struct * vm_next;

  11. /* for areas with inode, the circular list inode->i_mmap *//* for shm areas, the circular list of attaches *//* otherwise unused */ struct vm_area_struct * vm_nextt_share; struct vm_area_struct * vm_prev_share;/* more */ struct vm_operations_struct * vm_ops; 運算函數位址 unsigned long vm_offset; 作memory mapping struct inode * vm_inode; unsigned long vm_pte; /* shared mem */};

  12. Virtual memory model • Demand paging . Access not valid page table entry . Page fault report address and access type . Search vm_area_struct in a AVL tree to check illegal or legal virtual address (send SIGSEGV signal to process) . If legal virtual address then check type . Else the page fault is legal case note: differentiate between swap file and somewhere on disk

  13. Virtual memory model • How about O.S ? . Physical mode V.S Virtual addressing mode . Physical mode : no page tables / addr transfer . Linux OS run in physical mode

  14. Caches • Buffer Cache . the buffer cache is indexed via the device identifier and desire block number • Page Cache. Used to speed up access to images and data on disk . Bring page not through file system . Page read ahead

  15. Caches • Swap Cache . Only dirty pages are saved in swap file. no need to write it to the swap file if the page is already in the swap file

  16. Caches • Hardware Caches .TLB . To avoid three times of memory reference

  17. Page Allocation and Deallocation • Some data structure . Mem_map - all of physical pages in system are described by - a list of mem_map_t . Mem_map_t - describes a single physical page in system - field : count ->number of used the page age map_nr ->physical frame number

  18. Page Allocation and Deallocation • Some data structure . free _area vector - each element contains information about “block” of page - the block size upwards in power of two i.e. Free_area[0]=1 page per block Free_area[0]=2 page per block Free_area[0]=4 page per block <see figure>

  19. bit N is set if the N’th block is free Map is a point to bitmap to which keeps to track of allocated groups of page of this size

  20. Page Allocation and Deallocation • Buddy algorithm .Allocation- allocation code search free_area for requested size - follow the chain and check the map to find the free block - if no free block , search the twice size chain - break down this block ,then free blocks are queue on appropriate queue <ex: request 2 pages>

  21. Page Allocation and Deallocation • Buddy al gorithm .Deallocation - whenever a block of pages is freed ,the adjacent or buddy block of same size is checked to see if free . - if free then recombine into a bigger block free size. <ex frame 1 to be free >

  22. Page Allocation and Deallocation • Buddy al gorithm . .Conclude allocation tends to fragment memory to ”small” one deallocation tends to recombines pages into “bigger” one

  23. Swapping Out and Discarding pages • Kernel swap daemon (kswapd) . a special type of process,a kernel thread . Make sure that there enough pages in physical memory . Waiting for the kernel swap “timer” to periodically expire . Two scale : free_pages_high , free_page_low

  24. Swapping Out and Discarding pages • when to do ? .num of free pages fallen belowfree_page_high worse still free_page_low - below free_page_high ->swap free 3 pages - below free_page_low ->swap free 6 page .Timer

  25. Swapping Out and Discarding pages • Method .reducing the size of the buffer and page cache - discarding these pages dose not have too many harmful side effect because “caches”!! - check mem_map to see if some page is cached - shared pages are not considered and page can’t in both cache - if not enough cached page then consider shared page • Method . Swapping Out System V shared Memory pages

  26. Swapping Out and Discarding pages • Method .swapping out and discarding pages - look at each “process” in the system in turn to see if it is a good candidate for swapping - not swap or discard “shared” or “locked” page - not swap out all of the swappable pages of the process - decide by “age” in the mem_map_t (old)

  27. Swapping Out and Discarding pages • Method . Swapping Out System V shared Memory pagesi.e. kswapd remember which method that it used last time successfully

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